1. Field of the Invention
The present invention relates to a projection light source for use with a photolithographic projector. In particular, the present invention is an illumination system for use with a ring field projector.
2. Description of the Related Art
Ring field projectors are optical projectors used in photolithography, which is a process by which masks for circuit patterns are created on silicon for microprocessors and other computer chips. An example of a ring field projector is an Offner projector. In a ring field projector, a projection light source is used to create an arcuate band of light. This light is then used by the ring field projector to project a reticle of the desired circuit pattern onto the target silicon.
FIG. 1 is a view of a schematic representation of a conventional ring field projector. As shown in FIG. 1, a ring field projector includes trapezoidal mirror 2, concave mirror 3, and convex mirror 4. Reticle 1 carries an image of one layer for the desired circuit pattern. Arcuate band of light 6 from a projection light source (not shown) passes through reticle 1, and an arc-shaped image of the circuit pattern illuminated by arcuate band of light 6 is projected onto target silicon 7. Both reticle 1 and target silicon 7 are moved in coordination in a direction parallel to line A so as to cause arcuate band of light 6 to scan across the entire surface of reticle 1. As arcuate band of light 6 is scanned across reticle 1, an image of the entire circuit pattern is scanningly formed onto target silicon 7. Scanning speed, and thus overall efficiency, depends upon the amount of light projected through reticle 1 and the exposure required for target silicon 7.
After the circuit pattern is projected onto the target silicon, subsequent processing develops the image of the circuit pattern. The pattern is etched, doped or otherwise processed, thereby creating one layer for the desired circuit. The process of projection, development, and processing is repeated for each subsequent layer.
In order to be well-suited for this type of projector, a projection light source must produce an arcuate band of light with special optical characteristics. These optical characteristics include symmetrical pupil irradiance, symmetrical pupil distribution, uniform numerical aperture, and telecentricity.
Conventional projection light sources produce a region of light with the desired optical characteristics, but not the desired arcuate shape. In order to get the arcuate shape, the light passes through an arc-shaped mask in a screen. Masking, however, results in significant loss in light energy. Only a small amount of light passes through the arc-shaped mask, with the rest of the light being blocked by the mask. Since there is less light energy, exposure times are lengthened, resulting in poor efficiency.
Attempts have been made to increase the uniformity and telecentricity of projection light sources, for example by redistributing light energy through non-lens elements such as prisms and fly-eye arrangements. Although these methods result in increased uniformity and telecentricity, they also produce rectangular bands of light that are not matched to the arcuate field of the projector.
Attempts have been made to reformat the light using fiber optics. See Kanti Jain, Advances in Eximer Laser Lithography, 774 Lasers in Microlithography 115 (SPIE 1987). However, these attempts introduce non-uniformity in the image. Attempts have been made to reformat the light by reflecting off cylindrical mirrors. Although these attempts result in increased light energy, they also introduce non-preferred optical characteristics such as asymmetrical pupil irradiance and distribution, non-uniform numerical aperture, or decreased telecentricity. Accordingly, there is a need for a projection light source that generates an intense arcuate band of light (i.e., more intense than that produced by conventional systems) with symmetrical pupil irradiance, symmetrical pupil distribution, uniform numerical aperture, and a high degree of telecentricity.